In vehicles such as fork lift trucks, an internal combustion engine has been controlled by the use of a potentiometer connected with an operator's pedal to give an electric voltage signal to the control element of the combustion engine. Such an arrangement has a disadvantage in that potentiometers are very sensitive to shock and other mechanical influences in the zone of the pedal of the vehicle. In this area, only the strongest possible elements should be utilized. Also, potentiometers have the disadvantage that it is difficult to vary the magnitude of the signals in dependency upon the operation of the pedal, particularly when it is to be varied in a non-linear manner relative to pedal travel.
In certain applications for controlling the travel speed of a vehicle with electric drive with an operational element which can be operated at will, it is known to use a light responsive modulator to vary the magnitude of voltage or current for the drive. The known modulating device comprises a light source and a photosensitive element and a movable shutter between the light source and photosensitive element which controls the output voltage or current in accordance with the position of the shutter, the light received by the photosensitive element varying with the shutter position. While this type of device has been known in connection with the control of an electric drive, it has not been recognized that it will be desirable to use this type of element in the control of an internal combustion engine, particularly an internal combustion engine for driving a hydrostatic transmission as is commonly used in fork lift trucks. In the control of an internal combustion engine the use of the light responsive modulator enables the separation of electrically sensitive parts from the mechanical parts connected with the driving pedal and on which shock forces may be exerted in vehicles driven with internal combustion engines.